Build an HTTPS POST /webhooks/pdf endpoint that limits the request body, verifies the provider’s signature against the exact raw bytes, records a unique event ID, queues the work, and promptly returns a 2xx response. Do not download the PDF or perform slow business operations in the request handler. Providers may retry deliveries, so make processing idempotent.
What the handler should do
A webhook is an HTTP request from a service reporting an event, such as a completed or failed PDF job. The receiving endpoint should authenticate the request and durably accept it, not do the whole PDF workflow inline.
- Accept only the expected HTTP method and route over HTTPS.
- Limit and read the body once, preserving its exact bytes.
- Verify the signature using that provider’s documented signing scheme and secret.
- Parse and validate the verified event.
- Persist the event’s unique ID so retries cannot enqueue duplicate work.
- Enqueue the event durably, then return a successful 2xx response.
- Have a worker retrieve the PDF and perform storage and application updates.
This separation matters because a slow handler can fail to acknowledge delivery in time. OpenAI says webhook endpoints should respond quickly with a successful 2xx status. It retries deliveries that do not receive a successful response within a few seconds for up to 72 hours, with exponential backoff; duplicate copies can occur. See OpenAI’s webhook guide.
Implement a bounded Go handler
The following is a handler skeleton for Go’s standard net/http. The signature verifier, idempotency store, and queue are interfaces because signature headers, signing algorithms, event formats, and durable storage differ by provider. Connect them to the provider’s documented SDK or implementation before deploying; accepting every signature is not a safe substitute.
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package webhook
import (
"encoding/json"
"io"
"log"
"net/http"
)
type Event struct {
ID string `json:"id"`
Type string `json:"type"`
JobID string `json:"job_id"`
CreatedAt string `json:"created_at"`
}
type Verifier interface {
Verify(raw []byte, headers http.Header) error
}
type EventStore interface {
// InsertIfNew must be atomic and backed by a unique constraint on ID.
InsertIfNew(id string) (bool, error)
}
type Queue interface {
Enqueue(event Event) error
}
type Handler struct {
Verifier Verifier
Store EventStore
Queue Queue
}
func (h Handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
if r.Method != http.MethodPost {
http.Error(w, "method not allowed", http.StatusMethodNotAllowed)
return
}
r.Body = http.MaxBytesReader(w, r.Body, 1<<20)
defer r.Body.Close()
raw, err := io.ReadAll(r.Body)
if err != nil {
http.Error(w, "invalid or oversized body", http.StatusBadRequest)
return
}
if err := h.Verifier.Verify(raw, r.Header); err != nil {
http.Error(w, "invalid signature", http.StatusBadRequest)
return
}
var event Event
if err := json.Unmarshal(raw, &event); err != nil {
http.Error(w, "invalid JSON", http.StatusBadRequest)
return
}
if event.ID == "" || event.Type == "" || event.JobID == "" {
http.Error(w, "missing required event fields", http.StatusBadRequest)
return
}
inserted, err := h.Store.InsertIfNew(event.ID)
if err != nil {
http.Error(w, "temporarily unavailable", http.StatusServiceUnavailable)
return
}
if !inserted {
w.WriteHeader(http.StatusOK)
return
}
if err := h.Queue.Enqueue(event); err != nil {
// Do not acknowledge work that was not durably queued.
http.Error(w, "temporarily unavailable", http.StatusServiceUnavailable)
return
}
w.WriteHeader(http.StatusOK)
}
func main() {
// Construct Handler with provider-specific verifier, durable store,
// and durable queue, then register it at /webhooks/pdf.
log.Fatal(http.ListenAndServeTLS(":443", "cert.pem", "key.pem", nil))
}The sample’s main only illustrates TLS serving; it does not register a handler or provide the provider-specific adapters, so it is not a complete deployable service by itself. In an application, register a configured instance, for example with mux.Handle("/webhooks/pdf", handler), and use your deployment’s HTTPS termination and server configuration.
The 1 MiB limit follows the official OpenAI Go SDK example; it is an example limit, not a universal requirement for every PDF provider. Set a limit appropriate to the provider’s webhook payloads. The webhook normally contains event metadata rather than the PDF itself. Configure server read, write, header, and idle timeouts as well; do not let slow clients hold resources indefinitely.
Signature verification must precede JSON parsing
Signatures generally authenticate the precise bytes sent by the provider. Parsing and then reserializing JSON can change whitespace, key order, or escaping, causing legitimate verification to fail and potentially weakening validation. Read once, pass those same bytes plus headers to the verifier, and only unmarshal after verification succeeds.
Do not assume a header name, HMAC variant, timestamp format, or tolerance window from another provider’s example. Implement exactly the provider’s documented scheme. Reject absent or invalid signatures with a 4xx response, and keep signing secrets out of source code and logs.
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A database uniqueness constraint on the provider event ID is the core duplicate guard. The sample uses InsertIfNew; it must be atomic across concurrent requests, not a separate “check then insert.” Use the provider’s event or webhook ID where available. OpenAI identifies its webhook-id header as suitable for idempotency; other providers may put the identifier in the payload.
There is a failure window if the ID is committed but enqueueing fails: a retry then looks like a duplicate even though no job was queued. Avoid losing work by storing the event and an outbox/job record in one database transaction, then have a dispatcher publish pending work. Alternatively, use a durable queue and a carefully designed transactional handoff. The worker should also be idempotent, since queue delivery itself can be repeated.
Should the webhook download the PDF before returning 200?
Usually, no. Acknowledge after the verified event has been durably recorded and queued; let a worker retrieve the PDF. Downloading inline ties the provider’s delivery timeout to network latency, file size, storage availability, and downstream processing. If any of those are slow or fail, the provider can retry and create concurrent work.
The worker should validate that the event represents a supported success or failure type, fetch the PDF from the provider’s documented location, store it, and update the application’s job state. Treat download URLs as sensitive, avoid logging credentials or signed URLs, and handle expired URLs according to the provider’s status or regeneration API. If the queue or durable record is unavailable, return a non-2xx so the provider can retry rather than claiming receipt.
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OpenAI webhooks
OpenAI’s documented retry period is up to 72 hours with exponential backoff when a successful 2xx is not received or the endpoint does not respond within a few seconds. Use the webhook ID for deduplication and respond quickly after durable acceptance. Consult the current webhook documentation for signing and event details.
PDF Generator API
PDF Generator API documents asynchronous generation through POST /documents/generate/async and job-status retrieval through GET /documents/async/{jobId}; its requests use JWT authentication. Its 2026 documentation states limits of 2 requests per second and 60 requests per minute. Apply those limits when workers poll or retrieve results; do not assume webhook behavior or retry semantics beyond what its current documentation specifies. The Go client documents API version 4.0.28: PDF Generator API Go client.
PDFMonkey
PDFMonkey documents documents.generation.success, for which download_url is available, and documents.generation.failure, which includes failure_cause. Its webhook documentation describes automatic retries and signature verification and was last updated September 24, 2026. Match your event validation and verifier to its current documentation: PDFMonkey webhooks.
Test delivery and observe failures
A provider needs to reach your endpoint over the public internet for a real webhook test. OpenAI’s guide names ngrok and cloud development environments as options for local testing. Keep the temporary endpoint protected by signature verification, and avoid exposing a development service longer than needed.
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Log event IDs, event types, job IDs, verification outcomes, duplicate outcomes, queue outcomes, and worker failures. Do not log raw bodies or secrets by default. Track accepted, rejected, duplicate, and failed counts, plus queue delay and PDF retrieval failures. Retain enough event metadata to diagnose a failed job without retaining unnecessary personal or document data.
Troubleshooting common failures
- Valid deliveries fail signature checks: confirm that you verify the exact raw bytes, use the correct secret for the active environment, and follow the provider’s exact header and timestamp rules. Do not parse and reserialize first.
- Large or malformed body returns 400: check the configured body cap against the provider’s payload size and inspect server/proxy limits. Raise limits only as needed; the OpenAI Go example’s 1 MiB value is not necessarily right for another provider.
- Provider retries after a 2xx should have been sent: verify that the response is actually emitted promptly through your proxy and that it is a successful status. Long inline PDF downloads or synchronous business logic can exceed the provider’s response window.
- A duplicate event does not create a new job: that is expected for the same idempotency key. If the first attempt failed after inserting the ID but before durable enqueueing, repair the outbox/queue handoff rather than deleting the deduplication record blindly.
- Success events have no downloadable PDF: validate that you are handling the provider’s success event schema and use its documented URL or asynchronous job-status endpoint. Failure events may carry an error reason instead of a file URL.
- Local tests never arrive: make sure the local endpoint is publicly reachable through a tunnel or cloud development environment, and use the provider’s configured webhook URL.
Or skip the browser setup
ScreenshotNeo is a website screenshot API, not a PDF-generation webhook receiver, so it does not replace the Go endpoint or queue described above. For the separate task of capturing a rendered web page, its one-call API is:
curl -G "https://api.screenshotneo.com/v1/shot" -d access_key=YOUR_API_KEY --data-urlencode url=https://stripe.com -o shot.webpSee the ScreenshotNeo API documentation. It removes cookie banners, newsletter popups, and chat widgets before capture; bot checks, blank pages, and failed loads are not billed; an MCP server lets AI agents take screenshots; and 1,000 screenshots per month are free with no card, with paid plans starting at $5 for 3,000. Learn about ScreenshotNeo or sign up free for 1,000 screenshots a month with no card.
Frequently Asked Questions
Can I use the webhook event ID as my idempotency key?
Yes, when the provider supplies a stable unique event or webhook ID; confirm where that provider puts it and enforce uniqueness atomically.
Does a 2xx mean PDF processing is complete?
No. It should mean the event was durably accepted; the queued worker performs retrieval and processing afterward.
Quick Recap
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